The present invention relates to Class AB amplifier circuits and, more particularly, to bias circuitry for biasing the output push-pull transistor stage into partial conduction wherein the quiescent current flowing therethrough is made independent of the output transistor's base-to-emitter voltage.
There are many applications for monolithic integrated amplifiers to provide large currents to direct coupled loads. In order to reduce power consumption in order to prevent undesired heating of the die, the amplifier should be operated in a Class AB mode of operation. Class AB operation is desired over Class B operation because of the more faithful reproduction of the applied input signal at the output of the amplifier. In Class AB operation the amplifier will source and sink current with respect to the load coupled directly thereto.
Distortion at the cross-over point is a particular problem associated with Class AB operated amplifiers, i.e., cross-over is that point where the amplifier switches from sinking current to sourcing current and vice versa. Typically, cross-over distortion is severely limited by biasing the complementary transistor output stage of the amplifier into partial conduction by a small quiescent biasing current that is produced from a bias circuit which is matched to the output devices of the amplifier.
It is important that the bias circuit be matched to the output transistor to maintain low cross-over distortion. However, there may be different applications in which the characteristics of one or the other output transistor cannot be determined so that the bias circuitry cannot be matched thereto. For example, one such application arises with the use of Subscriber Loop Interface Circuits (SLICs) which provide balanced two wire to four wire telephone signal conversion. Most prior art SLICs comprise an integrated circuit having an output power stage which is realized by an external transistor to the SLIC that supplies drive current to the subscriber loop and which is operated in Class AB mode as is known.
The external power device utilized in such SLICs may be a single NPN/PNP power transistor or a Darlington amplifier. For instance, the MC34019 SLIC designed by Motorola, Inc. includes both a NPN and a PNP external transistor coupled between the SLIC and the subscriber loop which are operated in a Class AB mode. As the base-to-emitter voltage (V.sub.BE) of these external devices is either unknown or may vary from one device to another, there exists a need for producing a quiescent bias current on-chip of the SLIC to bias the external devices partially on, which bias current is made independent of the device characteristics.